Push-pull system



M. DISHAL ET AL.

PUSH-PULL SYSTEM Filed Aug. 4, 1945 March 1, 1949.

Patented Mar. 1, 1949 UNITED STATES ATNT OFFICE PUSH-PULL SYSTEM Application August 4, 1945, Serial No. 608,948

4 Claims.

The present invention relates to electron discharge tube circuits and more particularly to push-pull systems.

Generally in balanced modulators the voltages of the anodes are simultaneously varied in two ways. In one way the voltages on the anodes are inversly varied in relation to each other with respect to ground. In a second way the voltage of both anodes is varied in phase with respect to ground.

In certain systems, such modulators feed direct current push-pull amplifiers, the anodes of the modulator being directly coupled to the respective grids of the amplifier with the cathodes of the amplier grounded. Such systems have frequently proven unsatisfactory. This appears to be clue to the fact that the parallel shifting or cophasal variation of the modulators anode potentials with respect to ground changes the bias of the amplier tubes and shifts their operation so as to distort the push-pull signal.

An object of the present invention is the provision of an improved direct current amplifier.

Another object or the present invention is the provision of an improved amplifier in which the bias of the tubes therein is substantially independent of the parallel shifting with respect to ground of its input voltages.

Another object of the present invention is the provision of an improved modulator-amplifier system with the amplifier either D. C. or A. C. coupled to the modulator.

One of the diiculties encountered in balanced modulators is that due to the diierences in the characteristics of the tubes used therein. 'Where these tubes have diierent characteristics, it is difcult to make them balance with respect to ground.

Still another object of the present invention is the provision of a balanced modulator in which two tubes having slightly diierent characteristics may be empioyed and in which the balance with respect to ground may be maintained despite these differences.

Other and further objects of the present invention will become apparent and the invention will be best understood from the following description of embodiments thereof, reference being had to the drawings, in which:

Fig. 1 is a schematic diagram of a modulatoramplifier system embodying my invention; and

Fig. 2 is a schematic diagram of another Inodulator-ampliier system oi" the push-pull type embodying my invention.

Referring now to Fig. l, the modulatorampli- Iier system of the push-pull type there illustrated includes a balanced modulator l and a push-pull D. C. amplifier 2. A source 3 of voltages to be modulated is connected in push-pull through resistors l and 5 to the grids 6 and l respectively of a double triode tube 3, consisting of two tube sections Si and it respectively. Voltages from a source of modulating voltages il are applied through resistors l2 and i3 cophasally to grids 6 and 'I respectively. Anodes I4 and I5 are connected respectively to the grids i6 and l1 of a double triode i8 consisting of two sections i9 and 2t in the push-pull amplifier 2.

In accordance with our invention, provision iS made for discrepancies between the two sections 9 and l@ of the double triode 8 which tend to imbalance the circuit. Maintenance of balance in the circuit is particularly diiiicult when the modulating voltages produce wide variations in grid swing or bias as for example, a shift in the bias of the two sections between Zero and cut-01T. We have found that by adjusting both the normal bias of the sections and the value of the load resistors of each section, suitable balance can be obtained over the entire range of operation of the system. Accordingly means for making both of such adjustments are included in the balanced modulator of Fig. l.

A fixed source of biasing potential 2l may be arranged between the modulating voltage source and ground. 'The cathodes 22 and 23 of sections 9 and l0 respectively are connected to the opposite ends of the resistor of a potentiometer 2t, the arm of which is connected to ground. By varying the position of the arm of potentiometer 24, the relative bias on both sections can be varied, said resistor serving as the cathode resistor for both cathodes. If said resistor is large enough the biasing source 2| may be dispensed with.

Anode lli and anode i5 are connected to the opposite ends of a load resistance generally designated by the numeral 25 which consists of two resistors-2E and El, preferably of equal value, between which is arranged the resistor of a potentiometer 28 whose arm is connected to an intermediate point 29 of a source of D. C. potential 30 whose negative side is connected to ground. A voltage divider 3i is arranged in parallel with load resistance .'25, voltage divider 3i being comprised of resistors 32 and 33 between which is arranged the resistor of a potentiometer 3d, the arm of said potentiometer being connected to the positive side of the D. C. source 3Q.

Voltage divider 3l has a resistance many times that of load resistance 25, so that the eliective 3 load resistances of tube sections 9 and I9 are determined by 25. By Varying the position of the arm of potentiometer 28, the value of the load resistors of sections 9 and I9 are inversely varied with relation to each other.

In practice, the balanced modulator is adjusted in the absence of voltages from source 3, but with voltages, usually sinusodally varying, from source II. simultaneously adjusted until the trans-conductances of sections 9 and I9 track each other over the entire range of voltages from source I I. Usually after this operation, the voltages of anodes I4 and I5 are somewhat diierent, thus applying different biases to grids I6 and I1 of the amplifier tube sections I9 and 29. To correct this, the arm of potentiometer 34 is then adjusted. The positive side of D. C. source 39 is several times as high in potential as point 29 while the voltage divider 3I has a relatively large resistance. Therefore a small current is bled through divider 3l into the load resistance 25 and by adjustment of the arm, of potentiometer 34, this current is proportioned to vary the voltages of anodes I4 and I5 inversely and equalize them.

In the D. C. balanced amplifier 2, the cathodes 35 and 3S of sections I9 and 20 respectively are connected together, but in accordance with our invention, instead of being tied to ground while the grids vary cophasally with respect to ground, the cathodes float with the grids in respect to ground. For this purpose grid I6 is connected through a resistor 31, and grid I1 is connected through a resistor 38, to the common connection point 39 of cathodes 35 and 39. By the foregoing arrangement the cathodes 35 and 35 are not tied to ground but instead vary with relation to ground as do the grids I and I1 and the anodes I4 and I5, the cathode potential with respect to ground being equal to the average of the potentials of grids I'6 and I1 each with respect to ground. Resistors 31 and 38 are of such a value as to provide the bias required for the sections I9 and 20 respectively. The anodes 40 and 4I of sections I9 and 20 respectively are connected to output terminals 42 and 43 and are connected by means of load resistors 44 and 45 and line 46 to the positive side of the source 30.

In certain double triodes, a single cathode is used instead of two separate cathodes for each section. In such cases when tubes of such type are used in the modulator, cathode resistors as shown in a balanced modulator I of Fig. 1 cannot be employed but other means must be used for varying the bias on the two sections. modulator utilizing such tubes is illustrated in Fig. 2 in which the double triode 41 consists of two sections 48 and 49 having a single cathode 50. In balancing each of the said sections 48 and 49, the cathode 59 is connected to ground by a conductor 5I. The source of potential 2l may provide a xed bias but for the purposes of obtaining a variable bias, a source 52 of biasing voltages, which is balanced with respect to ground, is utilized. The opposite sides of the source 52 are connected through resistors 53 and 54 to the opposite ends of the resistor of a potentiometer 55, whose arm is connected to the grid 59 of section 49. A similar potentimeter 51 has its resistor cross-connected by crossed conductors 5B and 59 to the opposite ends of the resistor of potentiometer 55, the arm of potentiometer 51 being connected to grid 69 of section 48. The arms of potentiometers 55 and 51 are ganged together. It will be seen that when these arms are moved The arms of potentiometers 24 and 28 are A balanced in the same direction, the bias applied to grids 56 and 69 will be inversely varied, that is, the bias on one of said grids will be increased in a negative direction while the bias on the other of said grids will be increased in an opposite direction. This is accomplished without injecting the biasing voltages into the modulating source I I, which injection is usually undesirable. Thus our invention may be employed in adjusting the balance of two sections of a double triode in which only a single cathode is used for both sections. The rest of the system of Fig. 2 is similar to the system described with reference to Fig. 1, except that the case of A. C. coupling between modulator and amplifier is shown. A pair of condensers 5I are used to couple the modulator to the amplier. The arm of potentiometer 28 is connected to the positive side of a source of D. C. voltages 62 whose negative side is connected to ground. The arm of potentiometer 34 is connected to ground and to the negative side of a source of D. C. potential 63 whose positive side is connected between resistors 44 and 46.

While we have described above the principles of our invention in connection with specific apparatus, and particular modifications thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of our invention as defined in the accompanying claims.

We claim:

1. A balanced modulator and direct current amplier system comprising a balanced modulator for modulating energy from a rst source with energy from a second source including a iirst pair of electron discharge devices each having an anode, cathode and grid, means coupling said rst source to the grids in push-pull, means coupling said second source to the grids in parallel, means for inversely varying the bias of one of the grids with respect to its cathode relative to the bias of the other of the grids with respect to its cathode, a balanced amplifier including a second pair of electron discharge devices each having an anode, cathode and grid with the grids coupled to the anodes of said first pair of electron discharge devices and the cathodes connected with each other, resistance means interconnecting the grids of said second pair of electron discharge devices and having an intermediate point thereon connected with the cathodes of said second pair of electron discharge `devices a iirst and second load resistance mean-s for coupling said modulator and said amplifier, a capacitive means, :said first and second load resistance means connected in parallel by said capacitive means and interconnecting the plates of said modulator and the grids of said ampliiier, respectively, adjusting means for inversely varying the value of the load resistance of one of said devices with respect to the value of the load resistance of the other of said devices.

2. A system according to claim 1 wherein in said balanced modulator said bias-varying means includes a common cathode resistor connected at opposite end-s thereof to each of the cathodes of the first mentioned electron discharge devices, and means coupling a selected one of a number of intermediate points on said resistor to the negative side of the source of anode potential.

3. A system according to claim 1 wherein in said balanced modulator the first-mentioned pair of electron discharge devices are included within a single envelope and include a single cathode common to both of said devices, and said biasing means includes a source of biasing potentials 'balanced to ground, a pair or potenticmeters having their arms connected to the grids or" said devices in said balanced modulator and their resistances connected at oppo-site ends to said balanced source or biasing voltages, said cathode being connected to ground, and means for moving the arms of said potentiometer so as to inversely vary the effective bias applied to the grid of one of said electron discharge devices in said balanced modulator With respect to the grid of the other of -said electron discharge devices in said modulator.

4. A balanced modulator and direct current amplier system comprising a balanced modulator for modulating energy rom a first source with energy from a second source including a rst pair of electron discharge devices each having an anode, cathode and grid, means coupling said rst source to the grids in push-pull, means coupling said second source to the grids in parallel, means for inversely varying the bias of one of the grids With respect to its cathode relative to the bias of the other of the grids with respect to its cathode, a balanced direct current amplier including a second pair of electron discharge devices each having an anode, cathode and grid with the grids connected separately to the anodes of said rst pair of electron discharge devices and the cathodes connected With each other, resistance means interconnecting the grids of said second pair of electron discharge devices and having an intermediate point thereon connected with the cathodes of said second pair of electron discharge devices, a rst and ysecond load resistance means connected in parallel and interconnecting the plates of said modulator for coupling said modulator to said amplifier, adjustment means on each of said load resistance means for inversely varying the value of the load resistance of one of the said devices with respect to the value of the load resistance of one other of said devices, a direct current source, first connecting means for connecting said adjustment means of said first load resistance means to a point on said direct current source, vsecond connecting means for connecting said adjustment means or" said second resistance means to a point of higher potential on said direct current source than said rst connecting means, a third resistance means for interconnecting the plates of said amplier, an intermediate point on said third resistance means connected to said second connecting means.

MILTON DISI-IAL. JESSE S. LE GRAND.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES 'PATENTS Number Name Date 1,343,308 Carson June 15, 1920 1,778,085 Nyquist Oct. 14, 1930 2,019,481 Applegate Nov. 5, 1935 2.399.586 Toomim Apr. 30, 1946 

